1. Field of the Invention
The invention in general relates to the fabrication of layered superlattice materials, and more particularly to a fabrication method that provides ferroelectric integrated circuit devices containing thin films of layered superlattice materials possessing high-polarizability, low fatigue and low-leakage current characteristics by using a low-temperature rapid-temperature pulsing anneal.
2. Statement of the Problem
Ferroelectric compounds possess favorable characteristics for use in nonvolatile integrated circuit memories. See Miller, U.S. Pat. No. 5,046,043. A ferroelectric device, such as a capacitor, is useful as a nonvolatile memory when it possesses desired electronic characteristics, such as high residual polarization, good coercive field, high fatigue resistance, and low leakage current. Layered superlattice material oxides have been studied for use in integrated circuits. U.S. Pat. No. 5,434,102, issued Jul. 18, 1995, to Watanabe et al., and U.S. Pat. No. 5,468,684, issued Nov. 21, 1995, to Yoshimori et al., describe processes for integrating these materials into practical integrated circuits. Layered superlattice materials exhibit characteristics in ferroelectric memories that are orders of magnitude superior to those of PZT and PLZT compounds.
A typical ferroelectric memory in an integrated circuit contains a semiconductor substrate and a metal-oxide semiconductor field-effect transistor (MOSFET) electrically connected to a ferroelectric device, usually a ferroelectric capacitor. Layered superlattice materials currently in use and development comprise metal oxides. In conventional fabrication methods, crystallization of the metal oxides to produce desired electronic properties requires heat treatments in oxygen-containing gas at elevated temperatures. The heating steps in the presence of oxygen are typically performed at a temperature in the range of 800xc2x0 C. to 900xc2x0 C. for 30 minutes to two hours. As a result of the presence of reactive oxygen at elevated temperatures, numerous defects, such as dangling bonds, are generated in the single crystal structure of the semiconductor silicon substrate, leading to deterioration in the electronic characteristics of the MOSFET. Good ferroelectric properties have been achieved in the prior art using process heating temperatures at about 700xc2x0 C. to crystallize layered superlattice material. See U.S. Pat. No. 5,508,226, issued Apr. 16, 1996, to Ito et al. Nevertheless, the annealing and other heating times in the low-temperature methods disclosed in the prior art are in the range of three to six hours, which may be economically unfeasible. More importantly, the long exposure times of several hours in oxygen, even at the somewhat reduced temperature ranges, results in oxygen damage to the semiconductor substrate and other elements of the CMOS circuit.
After completion of the integrated circuit, the presence of oxides may still cause problems because oxygen atoms from a thin film of metal oxide layered superlattice material tend to diffuse through the various materials contained in the integrated circuit and combine with atoms in the substrate and in semiconductor layers, forming undesired oxides. The resulting oxides interfere with the function of the integrated circuit; for example, they may act as dielectrics in the semiconducting regions, thereby forming virtual capacitors. Diffusion of atoms from the underlying substrate and other circuit layers into the ferroelectric metal oxide is also a problem; for example, silicon from a silicon substrate and from polycrystalline silicon contact layers is known to diffuse into layered superlattice material and degrade its ferroelectric properties. For relatively low-density applications, the ferroelectric memory capacitor is placed on the side of the underlying CMOS circuit, and this may reduce somewhat the problem of undesirable diffusion of atoms between circuit elements. Nevertheless, as the market demand and the technological ability to manufacture high-density circuits increase, the distance between circuit elements decreases, and the problem of molecular and atomic diffusion between elements becomes more acute. To achieve high circuit density by reducing circuit area, the ferroelectric capacitor of a memory cell is placed virtually on top of the switch element, typically a field-effect transistor (hereinafter xe2x80x9cFETxe2x80x9d), and the switch and bottom electrode of the capacitor are electrically connected by a conductive plug. To inhibit undesired diffusion, a barrier layer is located under the ferroelectric oxide, between the capacitor""s bottom electrode and the underlying layers. The barrier layer not only must inhibit the diffusion of oxygen and other chemical species that may cause problemsxe2x80x94it must also be electrically conductive, to enable electrical connection between the capacitor and the switch. The maximum processing temperature allowable with current barrier technology is about 700xc2x0 C. At temperatures above 700xc2x0 C., the highest-temperature barrier materials degrade and lose their diffusion-barrier properties. On the other hand, the minimum feasible manufacturing process temperatures of layered superlattice materials used in the prior art is about 800xc2x0 C., which is the temperature at which deposited layered superlattice materials, such as strontium bismuth tantalate, are annealed to achieve good crystallization.
It is common in the art to use rapid thermal processing (xe2x80x9cRTPxe2x80x9d) before furnace annealing to improve ferroelectric or dielectric properties of deposited metal oxide thin films, in particular, of layered superlattice materials. Methods using RTP before oxygen annealing are described in U.S. Pat. No. 5,648,114, issued Jul. 15, 1997 to Paz de Araujo et al. and U.S. Pat. No. 5,825,057, issued Oct. 20, 1998 to Watanabe et al. The RTP disclosed in the prior art is typically conducted at a temperature of 700xc2x0 C. to 850xc2x0 C. for a hold time of about 30 seconds, followed by an oxygen furnace anneal at 800xc2x0 C. for 30 to 60 minutes. These process temperatures exceed the desired range described above, which does not exceed 700xc2x0 C.
For the above reasons, therefore, it would be useful to have a low-temperature method for fabricating layered superlattice materials in ferroelectric integrated circuits that minimizes the time of exposure to oxygen at elevated temperature, as well as reduces the maximum temperatures used.
The embodiments of the present invention reduce fabrication processing temperatures and reduce the time of exposure of the integrated circuit to oxygen gas at elevated temperature, while improving polarizability of the thin films of ferroelectric layered superlattice material.
An important feature of a method in accordance with the invention is a Rapid-Temperature Pulsing Anneal (xe2x80x9cRPAxe2x80x9d) technique. In an RPA, the temperature of a deposited thin film containing a plurality of metals is ramped up to a xe2x80x9chold temperaturexe2x80x9d at a rapid ramp rate, held at the hold temperature for a time period, the xe2x80x9cholding timexe2x80x9d, and then allowed to cool to a xe2x80x9ccool temperaturexe2x80x9d before a xe2x80x9cpulse sequencexe2x80x9d of ramping, holding, and cooling is repeated one or more times. The rapid ramp rate, the hold temperature, the holding time, and cool temperature may vary between pulse sequences. Typically, a liquid precursor is deposited on a substrate, dried to form a solid film, and then an RPA is conducted. An RPA technique in accordance with the invention may also be used in combination with a CVD deposition process.
The RPA method may be conducted in an RPA apparatus similar or identical to a conventional rapid thermal processing (xe2x80x9cRTPxe2x80x9d) apparatus. A significant difference between an RPA technique in accordance with the invention and an RTP technique is that the heating, or rapid rise in temperature, of an RPA occurs a plurality of times in a pulse-like manner, in contrast to the unitary heating and holding time of a typical RTP. Another related significant difference is that the temperature of the treated material is typically, but not always, allowed to cool between rapid-temperature pulsing. In accordance with the invention, the holding time at the hold temperature is typically 30 seconds, although it can vary between embodiments, and even between pulse sequences of a single embodiment, in a range of from 10 seconds to 60 minutes, preferably less than ten minutes, and most preferably in a range of from 20 seconds to five minutes. In accordance with the invention, an RPA technique is conducted in an oxygen-containing atmosphere to enhance formation of the metal oxide bonds in polycrystalline layered superlattice materials and other ferroelectric or dielectric compounds. It is contemplated, however, that an oxygen-free unreactive atmosphere may be used for a significant number of the pulsing sequences.
A method in accordance with the invention includes a series of two or more pulse sequences in which the temperature in the oven of the RPA apparatus is ramped up to a hold temperature. It is contemplated, however, that a plurality of hold temperatures may be used. The first hold temperature is relatively low, for example 400xc2x0 C. to 500xc2x0 C. The first heating pulse sequence, therefore, serves to decarbonize a deposited thin film, but avoids higher temperatures at which crystallization of the deposited atoms from the precursor compounds could occur. The holding time of the first heating pulse sequence may also be relatively long to achieve complete decarbonization. By avoiding crystallization in the initial heating pulse sequence, a method in accordance with the invention reduces or eliminates altogether the generation of the low temperature crystalline phases, which are referred to in the art as the xe2x80x9cfluorite phasesxe2x80x9d. The second and subsequent heating pulse sequences serve to crystallize deposited precursor atoms into the desired layered superlattice material or other ferroelectric or dielectric metal oxide material. The annealing of the metal oxide thin film by a rapid-temperature pulsing anneal, RPA, is typically followed by a conventional furnace anneal (xe2x80x9cFAxe2x80x9d). Depending on the hold temperature and holding time of the several heating pulse sequences of an RPA method, however, a conventional furnace anneal may be unnecessary after the RPA is conducted. At the higher hold temperatures in the second and subsequent pulse sequences, the rapid ramp rate of the temperature causes the crystalization process to proceed directly into the high temperature crystalline phase, thus reducing or eliminating altogether the generation of the low temperature crystalline, or xe2x80x9cfluoritexe2x80x9d, phases. The actual ramp rate of the rapid-temperature heating pulses is typically in the range of from 10xc2x0 C. to 100xc2x0 C. per second, preferably in a range of from 20xc2x0 C. to 60xc2x0 C. per second, and most preferably about 30xc2x0 C. per second. Typically, the hold temperature is the maximum temperature reached during an RPA pulse. During the rapid-temperature pulse sequences, the substrate may be cooled using conventional cooling techniques.
In accordance with the invention, the crystallization of layered superlattice material, or other ferroelectric or dielectric material, depends on numerous factors. These factors include: ramp rates, holding times, hold temperatures, cool temperatures, and oxygen-content of the RPA atmosphere, as well as the composition of the liquid precursor and the desired metal oxide material.
Ferroelectric layered superlattice materials, like the metal oxides SrBi2Ta2O9 (SBT) and SrBi2(Ta1-xNbx)2O9 (SBTN), where 0 less than x less than 1, are particularly useful in nonvolatile memory applications, such as in FeRAMs and nondestructible read-out ferroelectric FETs. Polycrystalline thin films of these layered superlattice materials, as well as other layered superlattice materials, may be fabricated in accordance with the invention.
In accordance with the invention, an RPA hold temperature suitable for forming a layered superlattice material is in the range of from 400xc2x0 C. to 750xc2x0 C., preferably less than 500xc2x0 C. in the first RPA sequence, and preferably between 600xc2x0 C. and 700xc2x0 C. in the second and subsequent RPA sequences.
It is a feature of the invention that it is not necessary to conduct an oxygen furnace anneal (xe2x80x9cFAxe2x80x9d) after the RPA. Thus, in certain embodiments in accordance with the invention, an RPA is the only heating technique performed in an oxygen-containing atmosphere to promote reaction and crystallization in the deposited thin film to form the desired polycrystalline layered superlattice material. Because heating of a ferroelectric or a dielectric metal oxide thin film by RPA is very effective compared with other heating techniques, such as furnace annealing, the maximum temperatures used in the complete fabrication process and the total time of exposure to oxygen at elevated temperatures are minimized.
After the RPA has been conducted, the substrate containing the layered superlattice material thin film may optionally be given an oxygen furnace anneal. An oxygen furnace anneal conducted after an RPA tends to increase the remanent polarization of the layered superlattice material.
In embodiments of the invention in which a liquid precursor is deposited as a liquid coating on a substrate, the RPA is typically preceded by a step of baking the coating on the substrate at a temperature not exceeding 400xc2x0 C., typically in an oxygen-containing ambient, typically in O2 gas.
In one aspect of the invention, the substrate comprises a first electrode, and the method includes steps of forming a second electrode on the thin film of layered superlattice material, after the RPA, to form a memory capacitor, and subsequently performing a step of post-annealing. Post-annealing may be conducted using an FA or rapid thermal processing (xe2x80x9cRTPxe2x80x9d) technique. In a preferred embodiment, the first electrode and the second electrode contain platinum and titanium. The post-anneal is conducted at a temperature in the range of from 500xc2x0 C. to 750xc2x0 C., preferably in oxygen at 650xc2x0 C. for 30 minutes. In one embodiment of the invention, the post-annealing is conducted in an oxygen-containing ambient, typically in O2 gas. Preferably, an electrically conductive barrier layer is formed on the substrate prior to applying the precursor coating.
The thin film of layered superlattice material typically has a thickness in a range of from 40 nm to 500 nm, preferably from 40 nm to 200 nm.
Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.